This invention relates to voice frequency communication systems and, more particularly, to the use of carbon microphones in voice frequency systems that operate at relatively low current levels.
Carbon microphones have heretofore been considered unusable for voice frequency transmission systems operating at low current levels because of unacceptably low signal-to-noise ratios, which low ratios are due to the fact that the gain of the carbon microphone falls off at low current levels while the noise remains substantially unchanged. In pursuit of a low current telephone station capable of operating with below 5 ma of battery current, for example, the major problem is the behavior of the carbon microphone under such conditions. However, the carbon microphone is still highly favored over other types of microphones because of its reliability and low cost. Moreover, continued use of carbon microphones is advantageous in view of their present almost universal use in many communication systems, such as telephone systems, for example.
In some prior art communications circuits, other types of microphones having linear gain characteristics have been substituted for the carbon microphone in low current applications. For example, telephone systems in low density rural areas require customer loops which extend many miles from a central office. The low operating currents encountered at the ends of these long customer loops have previously dictated the design of specialized handsets using electromagnetic transducers as transmitters. These electromagnetic transducer handsets have many drawbacks; a linear transmitter such as the electromagnetic transducer has mechanical as well as economic disadvantages. The same sensitivity which allows operation of the electromagnetic transducer at low current levels also permits pickup and reproduction of low level audio background noise. In contrast, this background noise is normally filtered out by the carbon microphone which is relatively insensitive to low level acoustics.
Other microphone types, such as the electret microphone, for example, have potential low current applications but are still in the experimental stage, and complete commercial feasibility has not yet been demonstrated.
In view of the advantages of carbon microphones as indicated above, and further, in view of the shortcomings of other types of microphones with known low current operating capabilities, the desirability of a carbon microphone adapted to operate in a low current environment is evident.
Moreover, the advantages to be gained from operating carbon microphones at low current levels in telephony applications are in no way restricted to long customer loop applications. Instead, for example, the universal operation of carbon microphone telephone handsets at reduced current levels, irrespective of loop lengths, could result in significant energy conservation. The electrical current requirements of the telephone handset carbon microphone needed to ensure good transmission quality are determining factors in power consumption, line voltage and transmission cable resistance. Accordingly, operation of the telephone handset at a uniform low current level on all lines of a telephone office would reduce total power consumption, cut backup battery requirements and allow fine gauge higher resistance loop cables.
One object of the invention, therefore, is to adapt a carbon microphone for effective operation under conditions of low power.
Another object is to combine the advantages of a carbon microphone with certain advantages of an electromagnetic microphone.
A related object is to reduce the power consumption of a telephone network.